Replace the EGR Valve. I had this same problem with my 454 chevy chassis Motorhome & pulled the heads & had the valves done, thinking I had a burned intake Valve! I still had the same problem after putting the heads back on! A leaking EGR Valve allows exhaust flame to get past the valve & ignite the fuel in the intake manifold. It only happens under hard acceleration, when there is a lot of pressure in the exhaust manifolds.